Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems related to seismic exploration and, more particularly, to mechanisms and techniques for synchronizing seismic source arrays.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, improving the resolution of images of the structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 may tow an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean or other body of water, or have spatial arrangements other than horizontal, such as at variable depths. The vessel 10 may also tow a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until, eventually, a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
In an effort to improve the resolution of the subsurface's image, an innovative solution (BroadSeis system of CGG, Massy, France) has been implemented based on broadband seismic data. The BroadSeis system may use Sentinel streamers (produced by Sercel, Nantes, France) with low noise characteristics and the ability to deploy in configurations that enable recording of an extra octave or more of low frequencies. The streamers are designed to record seismic data while being towed at greater depths and are quieter than other streamers. Thus, the receivers of these streamers would benefit from a marine broadband source array.
Such a source array that has a number of superior characteristics than existing source arrays is disclosed in patent application Ser. No. 13/468,589, filed on May 10, 2012, and assigned to the same assignee as the present application, the entire disclosure of which is incorporated herein by reference. This source array is illustrated in FIG. 2 as source array 50. The source array 50 may include multiple sub-arrays 60a-c, each having a corresponding float 52a-c. A plurality of source points 64 may be suspended from each float 52. However, different from existing sources, note that the source points 64 are suspended, from the same float, at two or more depths, and the configuration of the source points attached to one float may be different from the configuration of the source points attached to another float. For example, FIG. 2 shows that sub-array 60a has the higher depth source point behind the shallow source points along the direction Y, while the sub-array 60c has the higher depth source point between the shallow source points along the Y direction.
A source point of a source array may be an air gun or a cluster of air guns. An air gun stores compressed air and releases it suddenly underwater when fired. The released air forms a bubble (which may be considered spherical), with air pressure inside the bubble initially greatly exceeding the hydrostatic pressure in the surrounding water. The bubble expands, displacing the water and causing a pressure disturbance that travels through the water. As the bubble expands, the pressure decreases, eventually becoming lower than the hydrostatic pressure. When the pressure becomes lower than the hydrostatic pressure, the bubble begins to contract until the pressure inside again becomes greater than the hydrostatic pressure. The process of expansion and contraction may continue through many cycles, thereby generating a pressure (i.e., seismic) wave. The pressure variation generated in the water by a single source point, which is measured using a hydrophone or geophone located near the air gun as a function of time, is called the near-field signature and is illustrated in FIG. 3. A first pressure increase due to the released air is called primary pulse and it is followed by a pressure drop known as a ghost. Between highest primary pressure and lowest ghost pressure is a peak pressure variation (P-P). The pulses following the primary and the ghost are known as a bubble pulse train. The pressure difference between the second pair of high and low pressures is a bubble pressure variation Pb-Pb. The time T between pulses is the bubble period.
Single air guns are not practical because they do not produce enough energy to penetrate at desired depths under the seafloor, and plural weak oscillations (i.e., the bubble pulse train) following the primary (first) pulse complicates seismic data processing. These problems may be overcome by using arrays of air guns (i.e., a source array), generating a larger amplitude primary pulse and canceling secondary individual pulses by destructive interference.
A source array includes plural individual sources, as already discussed with regard to FIG. 2. Since the dimensions of the source array, including plural individual sources, are comparable with the generated wave's wavelength, the overall wave the source generates is directional, i.e., the shape of the wave, or its signature, varies with the direction until, at a great enough distance, the wave starts having a stable shape. After the shape become stable, the amplitude of the wave decreases inversely proportional with the distance. The region where the signature shape no longer changes significantly with distance is known as the “far-field,” in contrast to the “near-field” region where the shape varies. The far-field signature is an important quantity for the processing stage and, thus, it is desirable to know as accurately as possible its value.
The far-field signature has contributions from each individual source of the source array. To have a good far-field signature, it is desired that the primaries from each individual source arrive at a given far point, on a vertical below the source array, at the same time, i.e., they undergo constructive interference while the secondaries undergo destructive interference. If this condition is achieved, the far-field signature is of good quality. To achieve this far-field signature, the individual sources in the source array need to be synchronized, i.e., fired at such times that their primaries constructively interfere at the far point. The synchronization takes care of the different locations of the individual sources, sizes, types, volumes, age and maintenance conditions.
One such synchronization is disclosed in U.S. Pat. No. 8,174,927 (herein the “'927 reference”), the entire content of which is incorporated herein by reference. The '927 reference discloses using notional signatures of each individual source for aligning the firing of the source array. Further, the '927 reference discloses using the notional signatures with an attribute of the source for synchronizing the source array.
However, traditional methods assume that the source wavelet is constant for all shots in a survey (and from one survey to the next). This assumption can be invalid, especially when minor perturbations in wave height, gun pressure, array geometry and dropouts are present, which means that the true source signature varies from shot to shot.
Further, the stability and reliability of the far-field signature depends on the stability of each of the individual sources and of the source array's geometry. During a seismic survey, the individual sources' behavior may change (e.g., firing later or earlier than expected, than desirable, or at a smaller amplitude than nominally designed) and thus affect the far-field source signature. In practice, the gun controllers rely on a sensor called time-break (hereby called TB) installed inside each air-gun body to monitor the launch of each gun. However, for guns of different sizes, different models and/or different service time and maintenance conditions, the delay between the launch (electrical signal sent to gun and valve begins to open) and the actual shot (air goes out of the gun body and begins to generate the seismic wave) may vary.
Thus, despite the advances made by the seismic sources described above, calibrating and synchronizing seismic sources, particularly clusters of seismic sources, remains a challenge.